1. Field
This patent specification relates to electromagnetic measurements made in connection with boreholes. More particularly, this patent specification relates to methods and systems for making surface to borehole, borehole-to-surface and/or crosswell electromagnetic induction logging using metallic well casings as electrodes.
2. Background
Cross-well electromagnetic surveys have been used in the past to map formations in-between to wells in oil-field environment, and it is proven that it may provide useful information to the clients. There are many papers in this area, among the earliest ones are modeling studies in 1995 by B. R. Spies and T. M. Habashy, Sensitivity analysis of crosswell electromagnetics, Geophysics, Vol. 60, No. 3, P. 834-845, by David L. Alumbaugh and H. Frank Morrison, Theoretical and practical considerations for crosswell electromagnetic tomography assuming a cylindrical geometry, Geophysics, Vol. 60, No. 3, 1995, P. 846-870, and experimental studies by M. J. Wilt, D. L. Alumbaugh, H. F. Morrison, A. Becker, K. H. Lee and M. Deszcz-Pan, Crosswell electromagnetic tomography: system design considerations and field results, Geophysics, Vol. 60, No. 3, 1995, P. 871-885. A recent one is regarding a new generation crosswell EM system, see, L. DePavia, P. Zhang, D. L. Alumbaugh, C. Levesque, H. Zhang, and R. A. Rosthal, Next generation cross-well EM imaging tool, SPE, 2008.
There has been previous work on Electrical Resistance Tomography (ERT). See: W. D. Daily, A. L. Ramirez, Electrical resistance tomography using steel cased borholes as electrodes, U.S. Pat. No. 5,914,603, Jun. 22, 1999, W. D. Daily, C. Schenkel, and A. L. Ramirez, Electrical resistance tomography from measurements inside a steel cased borehole, U.S. Pat. No. 6,088,655, Jul. 11, 2000, and R. L. Newmark, Electrical Resistance Tomography using steel cased boreholes as electrodes, SEG 1999 expanded abstract. An array of steel cased boreholes are used as electrodes for current injection and electrical potential measurements to map subsurface the electrical resistivity distribution, and the electrical potential measurements can either be on surface, or inside a steel cased borehole.
U.S. Pat. No. 5,187,440 discusses measuring resistivity changes from within a first cased well to monitor fluids injected into oil bearing geological formations from a second cased well while passing electrical current between the two cased wells. However, the patent only discusses using cross well arrangement by injecting the current from fixed locations in each of two cased wells.